1. Field of the Invention
The present invention relates generally to data tracking, storage and retrieval systems and, more particularly, to data tracking, storage and retrieval systems that include steerable optics.
2. Background Art
In data recording and retrieval systems that use a moving media having a varying material characteristic, detectable variations from previously encoded media locations may be retrieved using reflected incident light. Such variations may also be used to provide servo control signals for following previously recorded data tracks. For example, in a magneto-optical storage system, using a magneto-optical (MO) recording material deposited on a rotating disk, information may be recorded on the disk as spatial variations of magnetic domains. During readout, the magnetic domain pattern modulates an optical polarization, and a detection system converts a resulting signal from optical to electronic format.
In one type of magneto-optical storage system, a magneto-optical head assembly is located on a linear actuator that moves the head along a radial direction of the disk to position the optical head assembly over data tracks during recording and readout. A magnetic coil is placed on a separate assembly on the head assembly to create a magnetic field that has a magnetic component in a direction perpendicular to the disk surface. A vertical magnetization vector of polarity (opposite to that of the surrounding magnetic material of the disk medium) is recorded as a mark indicating zero or a one by first focusing a beam of laser light to form an optical spot on the disk. The optical spot functions to heat the magneto-optical material to a temperature near or above a Curie point (i.e., a temperature at which the magnetization may be readily altered with an applied magnetic field). A current passed through the magnetic coil orients the spontaneous vertical magnetization vector either up or down. This orientation process occurs in the region of the optical spot where the temperature is suitably high. The orientation of the magnetization mark is preserved after the laser beam is removed. The mark is erased or overwritten if it is locally reheated to the Curie point by the laser beam while the magnetic coil creates a magnetic field in the opposite direction.
Information is read back from a particular mark on the disk by taking advantage of the magnetic Kerr effect to detect a Kerr rotation of the optical polarization that is imposed on a reflected beam by the magnetization at the mark of interest, the magnitude of the Kerr rotation being determined by the material's properties (embodied in the Kerr coefficient). The sense of the rotation is measured by established differential detection schemes as being clockwise or counter-clockwise depending on the direction of the spontaneous magnetization at the mark of interest.
Conventional magneto-optical heads, while presently providing access to magneto-optical disks with areal densities on the order of 1 Gigabit/in.sup.2, tend to be based on relatively large optical assemblies which make the physical size and mass of the head rather bulky. Consequently, the speed at which conventional magneto-optical heads are mechanically moved to access new data tracks on a magneto-optical storage disk is slow. In addition, the physical size of the prior art magneto-optical heads limits the spacing between magneto-optical disks. Consequently, because the volume available in standard height disk drives is limited, magneto-optical disk drives have not been available as high capacity commercial products. For example, a commercial magneto-optical storage device presently available provides access to only one side of a 130 mm double sided 2.6 ISO gigabyte magneto-optical disk, a 40 ms disk access time, and a data transfer rate of 4.6 MB/Sec.
N. Yamada (U.S. Pat. No. 5,255,260) discloses a low-profile flying optical head for accessing an upper and lower surface of plurality of optical disks. The flying optical head disclosed by Yamada describes an actuating arm having a static (i.e., fixed relative to the arm) mirror or prism mounted thereon, for delivering light to and receiving light from a phase-change optical disk. While the static optics described by Yamada provides access to both surfaces of a plurality of phase-change optical disks contained within a fixed volume, use of the optics disclosed by Yamada is inherently limited by how small the optics can be made. Consequently, the number of optical disks that can be manufactured to function within a given volume is also limited. Another shortcoming relates to the use of static optics. This approach imposes a limit on track servo bandwidth by requiring the entire optical head assembly to move in order to change the location of a focused optical spot. This same limitation applies to the flying magneto-optical head disclosed by Murakami et al. in U.S. Pat. No. 5,197,050. In general, the larger the mass of the element used to perform fine track serving, the lower the servo bandwidth becomes and the lower the track density that can be read or written.
A method for moving a folding prism or mirror with a galvanometer actuator for fine tracking has been disclosed by C. Wang in U.S. Pat. No. 5,243,241. The galvanometer consists of bulky wire coils and a rotatable magnet mounted on a linear actuator arm attached to a flying magneto-optical head, but not mounted on the slider body itself. This design limits the tracking servo bandwidth and achievable track density due to its size and weight. Its complexity also increases the cost and difficulty of manufacture.
Miniature torsional scanning mirrors have been described, viz, "Silicon Torsional Scanning Mirror" by K. Petersen, IBM J. Res. Develop., Vol. 24, No. 5 September 1980, pp. 631-637. These mirrors are generally prepared using procedures developed in the semiconductor processing arts. Petersen describes a torsion mirror structure having a 134 .mu.m thick silicon wafer defining a distal frame suspending a central silicon mirror element suspended by lateral torsion members therebetween. The lateral mirror dimensions are about 2.1 by 2.2 mm. The mirror is bonded over a 7 to 10 .mu.m deep etched well in a glass slide substrate, having evaporated electrodes deposited therein. The mirror is rotationally deflected by voltages applied between the mirror and the electrodes by connecting wires. Scanning angles of up to 0.2.degree. at a resonant operating frequency of up to 15 kHz were reported. The size and mass of the mirror limited higher operating frequency. Also, mirror distortion caused by the high dynamic torque (i.e., peak angular acceleration) at higher frequency was a limiting factor. The high mechanical Q of prior art mirrors hinders the ability to achieve precise angular deflection vs. voltage characteristics when operating in a range close to the resonant frequency. In the prior art, control of the mirror at large deflection angles becomes problematic due to the spontaneous deflection of the mirror tip to the substrate at a critical control angle when the tip of the electrostatically deflected element approaches within about 1/3 of the way down into the etched well. See "Silicon as a Mechanical Material", K. Petersen, Proceedings of the IEEE, VOL. 70, No. 5, May 1982, pp. 446-447.
Conventional multiple platter Winchester magnetic disk drives of the prior art use a set of respective suspensions and actuator arms that move in tandem as one integral unit. Because each flying magnetic head of such an integral unit is fixed relative to another flying magnetic head, during track following of a particular magnetic disk surface simultaneous track following of another magnetic disk surface is not possible.
What is needed is an improved optical head that is compact and that allows an increase in the number of storage disks that can be placed within a given volume as compared to the prior art. The improved optical head should preferably provide a high numerical aperture, a reduced head mass, a very high resonance frequency tracking servo device thus producing a very fine track servo bandwidth, and be relatively easy to manufacture. Additionally, the flying optical head should improve upon optical disk drive access times and data transfer rates and access to and use of storage disk tracks.